The possibility of producing ethanol from cellulose-containing lignocellulosic feedstocks such as wood, cultivated crops like switch grass and waste agricultural fibers such as wheat straw and oat hulls has received much attention due to the availability of large amounts of feedstocks, the desirability to avoid burning or landfilling cellulosic waste materials, and the cleanliness of ethanol as a fuel compared to gasoline.
The efficient conversion of cellulose from lignocellulosic material into glucose and the subsequent fermentation of glucose to ethanol represents a major challenge. Cellulose, which is the primary constituent of lignocellulosic fibers, consists of a crystalline structure that is very resistant to breakdown, as is hemicellulose, the second most prevalent component. The conversion of lignocellulosic fibers to ethanol requires: 1) liberating cellulose and hemicellulose from lignin or increasing the accessibility of cellulose and hemicellulose within the lignocellulosic feedstock to cellulase enzymes, 2) depolymerizing hemicellulose and cellulose carbohydrate polymers to free sugars and, 3) fermenting the mixed hexose and pentose sugars to ethanol.
Methods used to convert cellulose to glucose typically include acid hydrolysis (reviewed by Grethlein; Chemical Breakdown Of Cellulosic Materials, J.APPL.CHEM. BIOTECHNOL. 28:296-308 (1978)). Acid hydrolysis involves the use of either concentrated or dilute acids. The concentrated acid process uses 72% by weight sulfuric acid or 42% by weight hydrochloric acid at room temperature to dissolve the cellulose, followed by dilution to 1% acid and heating to 100° C. to 120° C. for up to three hours to convert cellulose oligomers to glucose monomers. This process produces a high yield of glucose, but the recovery of the acid, and the specialized construction materials required for the apparatus to carry out this process are serious disadvantages. Similar problems are encountered when concentrated organic solvents are used for cellulose conversion.
U.S. Pat. No. 5,536,325 describes a two-step process for the acid hydrolysis of lignocellulosic material to glucose. The first (mild) step depolymerizes the hemicellulose to xylose and other sugars. The second step depolymerizes the cellulose to glucose. Even though the process uses low levels of acid, the amount of acid required for the hydrolysis of the feedstock is substantial, and the resulting yield of glucose from cellulose is poor.
Other methods for converting lignocellulosic material to ethanol use a multistep procedure in which the lignocellullosic material is first pretreated at high temperature and pressure and often in the presence of various chemicals. This pretreatment process is thought to increase the accessibility of cellulose within the lignocellulosic fibers for subsequent conversion steps. As a large portion of the cellulose within untreated lignocellulosic material is unaccessible for subsequent enzymatic conversion steps, the efficiency of this pretreatment phase can profoundly influence the overall efficiency and commercial application of the entire conversion process.
Among the more successful pretreatment processes for the conversion of lignocellulosic feedstock into glucose, are dilute acid prehydrolysis processes. One effective dilute acid hydrolysis pretreatment is steam explosion as disclosed in U.S. Pat. No. 4,461,648 (Foody process, which is herein incorporated by reference). In the Foody process, biomass is loaded into a vessel known as a steam gun. Up to 1% acid is optionally added to the biomass in the steam gun or in a presoak. The steam gun is then filled very quickly with steam and held at high pressure for a set length of time, known as the cooking time. Once the cooking time elapses, the vessel is depressurized rapidly to expel the pretreated biomass. As a result of the rapid depressurization, the Foody process has been termed “steam explosion”. Specific parameters for steam explosion pretreatments are set out in U.S. Pat. No. 4,461,648; and Foody, et al, Final Report, Optimization of Steam Explosion Pretreatment, U.S. DEPARTMENT OF ENERGY REPORT ET230501 (Apr. 1980), which are herein incorporated by reference.
U.S. Pat. No. 4,237,226 describes the dilute-acid pretreatment of oak, newsprint, poplar, and corn stover by a continuous plug-flow reactor, a device that is similar to an extruder. Rotating screws convey a feedstock slurry through a small orifice, where mechanical and chemical action break down the fibers to increase the accessibility to cellulose.
One shortcoming of dilute acid prehydrolysis is the high acid requirement. For a clean feedstock, such as washed hardwood, the sulfuric acid demand is 0.5% to 1% of the dry weight of the feedstock. For agricultural fibers, which can contain high levels of silica, salts, and alkali potassium compounds from the soil, the acid demand is about 10-fold higher, reaching 5% to 7% by weight of feedstock. This adds significant additional cost to the process. A second drawback of using large amounts of acids in a prehydrolysis process is that an acidified feedstock must be neutralized to a pH between about 4.5 and about 5 prior to enzymatic hydrolysis with cellulase enzyme. The amount of caustic soda used to neutralize acidified feedstock is proportional to the amount of acid used to acidify the feedstock. Thus, high acid usage results in high caustic soda usage, which further increases the cost of processing lignocellulosic feedstock to ethanol.
Another drawback of steam explosion and other dilute acid pretreatment processes is that the while the treatment conditions significantly increase accessibility to cellulose, these same conditions result in the destruction and loss of xylose. Xylose is not as stable as the other sugars and has a tendency to break down in acid pretreatment conditions. The breakdown of xylose decreases the overall sugar yield that can be obtained from lignocellulosic feedstocks and this in turn decreases ethanol yield.
U.S. Pat. No. 5,198,074 and U.S. Pat. No. 4,857,145 (which are herein incorporated by reference) disclose washing chiped feedstock with water prior to removing a soluble fraction, used for the production of ethanol, and a fibre fraction for use in pulp and paper production. There is no disclosure of the use of the pretreated fraction for the production of xylose or ethanol.
U.S. Pat. No. 5,846,787 discloses processes for pretreating cellulosic materials prior to enzymatic conversion with cellulases. The process involves heating the cellulosic materials in water at a temperature at or above their glass transition temperature while maintaining the pH of the reaction medium in a range that avoids autohydrolysis of the cellulosic materials. The method is performed in place of a dilute acid or steam explosion pretreatment process. The water used in the pretreatment process of U.S. Pat. No. 5,846,787 is heated under pressure to a temperature in excess of 100° C., and thus requires a significant amount of energy to heat the water to such temperatures and is expensive and inefficient.
U.S. Pat. No. 4,326,892 discloses a method of improving the recovery of sugar from sugar beets. Sugar beets contain primarily sucrose, with little cellulose. The method comprises washing sugar beets to remove impurities therefrom, removing the outer layer of the of the washed sugar beets, slicing the sugar beets and extracting the slice sugar beets with an aqueous solution. After this washing, the relative amount of cellulose in the resulting beet pulp is increased. There is no suggestion that the pulp obtained by this process may be used for the production of xylose or ethanol as described herein.
U.S. Pat. No. 6,251,643 discloses a method for processing biomass using a screw press. In this method, following separation of a liquid phase from the solid phase, the solid phase is heated under pressure, to a temperature of 100-170° C., to produce a vapour treated phase of solids which is then further processed. The use of high temperatures to produce the vapour treated phase results in the denaturing of any protein component within the solid phase. Furthermore, the process described in this document results in a relatively low yield of ethanol from biomass (180-250 liters ethanol per metric ton dry material).
The publication entitled Wet Milling of Grain for Alcohol Production by Lyons et al., in Chapter 1 of The Alcohol Textbook 1995 (Nottingham University Press) discloses wet milling of corn in alcohol production. The corn kernel comprises approximately 70% starch and contains little cellulose. After starch removal, the resulting corn fiber is high in cellulose. The reference teaches soaking clean corn in tanks for about 20 to 40 hours with steep water/acid containing about 1600 ppm SO2, at a temperature of about 52° C. This steeping is the first step in removing starch from corn. However, the reference does not teach the use of the corn fiber for the production of xylose or ethanol as described herein.
The publication entitled “Separation Processes” by C. Judson King (1980, McGraw-Hill Book Company) discloses processing of sugar cane. Sugar cane is primarily sugar, with little cellulose. The reference teaches washing sugar cane with jets of water to remove field debris, followed by chopping the sugar cane into short sections, passing the sections through high pressure rollers and adding water to remove the available sugar. The sugar solution is processed and refined into raw sugar, black molasses and other products. After sugar removal, the remaining cane pulp (bagasse) is high in cellulose content. There is no suggestion that the remaining bagasse maybe used for xylose or ethanol production as outline herein.
Jenkins et al., (Measurements of the Fouling and Slagging Characteristics of Banagrass (Pennisetum purpureum) Following Aqueous Extraction of Inorganic Constituents. In: Making a Business from Biomass in Energy, Environment, Chemicals, Fibers and Materials. Proceedings of the 3rd Biomass Conference of the Americas, Montreal, Quebec, Canada, August 24-29. Pergamon (Elsevier Science)) discloses the washing of biomass fuel by aqueous extraction to control slagging and fouling in combustion systems burning banagrass. There is no suggestion that the washed lignocellulosic biomass maybe used for xylose or ethanol production as described herein.
Methods that improve xylose yield during acid hydrolysis improve the cost efficiency of converting lignocellulosic feedstock to sugars. Furthermore, methods that reduce the amount of acid required for dilute acid pretreatments increase the cost efficiency of ethanol production.
There is a need in the art to reduce the amount of acid which must be used in a pretreatment process. Further, there is a need in the art to increase xylose yield from lignocellulosic feedstocks subjected to pretreatment processes.
It is an object of the present invention to overcome disadvantages of the prior art.
The above object is met by a combination of the features of the main claims. The sub claims disclose further advantageous embodiments of the invention.